The U.S. infant mortality rate dropped last year to 9.7 per 1,000 live births, its lowest level ever, Secretary of Health and Human Services Louis W. Sullivan announced yesterday. But the rate remains higher than in many other developed nations.

"Although we've made progress in reducing this nation's infant mortality, we must do better," Sullivan said. "In particular, we must continue to direct prevention, education and health-care opportunities to our minority and poor citizens, where infant mortality strikes hardest."

Japan has the world's lowest infant mortality rate -- 5 per 1,000. Finland, Sweden, Norway, Switzerland, Canada and many other nations also have lower rates than the United States.

The National Center for Health Statistics reported that 4,021,000 babies were born in the United States last year. The 9.7 mortality rate means that nearly 40,000 died before their first birthday. The infant mortality rate in 1988 was 9.9; in 1960, it was 26.

The center said the life expectancy of persons born in the United States last year is 75.2 years, the highest ever and up from 74.9 in 1988.

For both infant mortality and life expectancy, the statistics are far more favorable for whites than for blacks. In 1987, the latest year for which a racial breakdown is available, the white infant mortality rate was 8.6 per 1,000, but the rate for blacks was more than double, 17.9 per 1,000.

Similarly, while whites born in 1989 had a life expectancy of 75.9 years, the life expectancy of blacks was only 69.7 years.

Rae Grad, executive director of the National Commission to Prevent Infant Mortality, said nutrition and health care for the mother during pregnancy are major factors in determining whether a child will die in its first year.

"We are pleased at the drop in infant mortality, though the figure is still too high. But the gains that we are making come at a very high cost. We are saving a lot of babies by using expensive high technology after they are born with disabilities or life-threatening impairments," Grad said. "We could do much better by investing in more prevention such as universal access to health care and nutrition in pregnancy, particularly through programs such as Medicaid or private health insurance and the Women-Infant-Children food program."

A draft report by a White House task force headed by James O. Mason, HHS assistant secretary for health, reached basically the same conclusions, declaring that the nation already knows how to save 10,000 of the 40,000 children who die in the first year of life: by ensuring that "the women at the greatest risk of having an unhealthy baby have access to high-quality primary health and social services," particularly to prevent low birth weight in babies, the primary cause of infant death.

The draft report said smoking, drinking, drug use, poor nutrition and poor health care during pregnancy, along with other conditions often associated with poverty, are risk factors for low birth-weight babies.

In yesterday's announcement, the National Center for Health Statistics said overall death rates in the United States, adjusted for age, reached their lowest level in 1989, although deaths caused by diabetes increased and those caused by AIDS rose to 21,000 from 16,000 in 1988. AIDS became the 11th leading cause of death in the United States, up from 15th in 1988.

The center also said birth and fertility rates were the highest since the early 1970s. It also reported there were 2.4 million marriages and 1.16 million divorces last year.

The District recently reported that its provisional infant mortality rate was 23.1 per 1,000 live births in 1989. In 1987, the last year for which the center has final figures, the D.C. rate was 19.3. Maryland's rate was 11.5, and Virginia's was 10.2.

The District, a central city with a large low-income population, cannot fairly be compared with whole states, which also include affluent suburban populations. A comparison with other cities is more apt, according to experts on statistics. In 1987, Baltimore's rate was 19.2, Chicago's 16.6, Detroit's 19.7 and New York's 12.7, the center said. In all of those cities the mortality rate for black infants was higher than the overall rate.

Staff researcher James Schwartz contributed to this report.